Hi again! On Mo, 2021-06-28T13:34-0400, Paul Moore wrote: > On Mon, Jun 28, 2021 at 1:13 PM Thomas Weißschuh <li...@weissschuh.net> wrote: > > On Mo, 2021-06-28T12:59-0400, Paul Moore wrote: > > > On Mon, Jun 28, 2021 at 9:25 AM Thomas Weißschuh <li...@weissschuh.net> > > > wrote: > > > > > > > > Hi everyone, > > > > > > > > there does not seem to be a way to access the AUDIT_ARCH_ constant that > > > > matches > > > > the currently visible syscall numbers (__NR_...) from the kernel uapi > > > > headers. > > > > > > Looking at Linus' current tree I see the AUDIT_ARCH_* defines in > > > include/uapi/linux/audit.h; looking on my system right now I see the > > > defines in /usr/include/linux/audit.h. What kernel repository and > > > distribution are you using? > > > > I am using ArchLinux and also have all these defines. > > > > > > Questions: > > > > > > > > Is it really necessary to validate the arch value when syscall numbers > > > > are > > > > already target-specific? > > > > (If not, should this be added to the docs?) > > > > > > Checking the arch/ABI value is important so that you can ensure that > > > you are using the syscall number in the proper context. For example, > > > look at the access(2) syscall: it is undefined on some ABIs and can > > > take either a value of 20, 21, or 33 depending on the arch/ABI. > > > Unfortunately this is rather common. > > > > But when if I am not hardcoding the syscall numbers but use the > > __NR_access kernel define then I should always get the correct number for > > the > > ABI I am compiling for (or an error if the syscall does not exist), no? > > Remember that seccomp filters are inherited across forks, so if your > application loads an ABI specific filter and then fork()/exec()'s an > application with a different ABI you could be in trouble. We saw this > some years ago when people started running containers with ABIs other > than the native system; if the container orchestrator didn't load a > filter that knew about these non-native ABIs Bad Things happened.
My application will not be able to spawn any new processes. It is limited to write() and exit(). Also this is a low-level system application so it should always be compiled for the native ABI. So this should not be an issue. > I'm sure you are already aware of libseccomp, but if not you may want > to consider it for your application. Not only does it provide a safe > and easy way to handle multiple ABIs in a single filter, it handles > other seccomp problem areas like build/runtime system differences in > the syscall tables/defines as well as the oddball nature of > direct-call and multiplexed socket related syscalls, i.e. socketcall() > vs socket(), etc. For a larger application this would be indeed my choice. But for a small application like mine I don't think it is worth it. libseccomp for example does provide a way to get the native audit arch: `uint32_t seccomp_arch_native(void);`. It is implemented by ifdef-ing on various compiler defines to detect the ABI compiled for. I'd like the kernel to provide this out-of-the box, so I don't have to have the same ifdefs in my application(s) and keep them up to date. I found that the kernel internally already has a definition for my usecase: SECCOMP_ARCH_NATIVE. It is just not exported to userspace. > > > Checking the arch/ABI value is also handy if you want to quickly > > > disallow certain ABIs on a system that supports multiple ABI, e.g. > > > disabling 32-bit x86 on a 64-bit x86_64 system. > > > > > > > Would it make sense to expose the audit arch matching the syscall > > > > numbers in > > > > the uapi headers? > > > > > > Yes, which is why the existing headers do so ;) If you don't see the > > > header files I mentioned above, it may be worth checking your kernel > > > source repository and your distribution's installed kernel header > > > files. > > > > I do see constants for all the possible ABIs but not one constant that > > always > > represents the one I am currently compiling for. > > The same way the syscall number defines always give me the syscall number > > for > > the currently targeted ABI. > > I'm sorry, but I don't quite understand what you are looking for in > the header files ... ? It might help if you could provide a concrete > example of what you would like to see in the header files? I want to do something like the follwing inside my program to assemble a seccomp filter that will be loaded before the error-prone parts of the application will begin. 1: BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_arch), 2: BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, SECCOMP_ARCH_NATIVE, 0, $KILL) 3: BPF_STMT(BPF_LD | BPF_W | BPF_ABS, syscall_nr), 4: BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_write, $ALLOW, $KILL), In line 4 I can already have the kernel headers provide me the correct syscall number for the ABI my application is compiled for. For line 2 however I need to define AUDIT_ARCH_CURRENT on my own instead of having a kernel header provide the correct value. Thomas -- Linux-audit mailing list Linux-audit@redhat.com https://listman.redhat.com/mailman/listinfo/linux-audit
